In the world of mapping, unique identification of geographic objects is paramount.
A Location Reference (LR) is a unique identification of a geographic object. Proposed International Standard, ISO 17572 Location Referencing for Geographic Databases, describes location referencing in great detail, with Part 3 covering Dynamic Location Referencing (the AGORA-C method). Generally, in a digital world, a geographic object can be represented by a feature in a geographic database. An example of a commonly known Location Reference is a postal address of a house. Examples of object instances include a particular exit ramp on a particular motorway, a road junction or a hotel. For efficiency reasons, Location References are often coded. This is especially significant if the Location Reference is used to transmit information about various objects between different systems. For Intelligent Transportation Systems (ITS), many different types of real world objects will be addressed. Among these, Location Referencing of the road network or components thereof, is a particular focus.
Communication of a Location Reference for specific geographic phenomena, corresponding to objects in geographic databases, in a standard, unambiguous manner can be a vital part of an integrated ITS system, in which different applications and sources of geographic data will be used. Location Referencing Methods (LRM, methods of referencing object instances) differ by applications, by the data model used to create the database, or by the enforced object referencing imposed by the specific mapping system used to create and store the database. A standard Location Referencing Method allows for a common and unambiguous identification of object instances representing the same geographic phenomena in different geographic databases produced by different vendors, for varied applications, and operating on multiple hardware/software platforms. If ITS database technology is to become widespread, data reference across various applications and systems must be possible. Information prepared on one system, such as traffic messages, must be interpretable by all receiving systems. A standard method to refer to specific object instances is essential to achieving such objectives.
Presently, most systems utilize what is termed “pre-coded location references” or “pre-code profiles”. In the pre-coded reference model, each system (the sending system and the receiving system) has its own pre-coded map table (or location table) with digital geographical data representing often used locations, or geographical objects, such as buildings, streets, etc. In this way, if a sender wished to reference a house, for instance, it would retrieve from its local map table the pre-coded geographical data representing the location of the, e.g., house and send the data to the receiver. The receiver would utilize the received data to retrieve from its local map table the information regarding that location it needed to perform the desired function. Pre-coded location referencing is utilized in the Radio Data System—Traffic Message Channel (RDS-TMC), a technology for delivering traffic and travel information to drivers.
The pre-coded location reference model, while having the advantage of the conciseness of the code, causes numerous problems, however. First, the map tables at the sender and the receiver need to be kept in sync so that there is no ambiguity in the location information. This is time-consuming and expensive and, many times, cannot be accomplished (such as when either the sender or the receiver has an older version of the map table) such that different map tables and data are in use concurrently causing obvious problems. Secondly, the map table of the sender may be provided by a different map table provider than the map table provided to the receiver such that the map tables are, again, different, causing the same types of problems. Finally, the pre-coded location referencing model has a limited number of addressable locations.
Consequently, pre-coded location referencing is not the preferred location referencing model; rather, dynamic location referencing is becoming the preferred standard. One example of dynamic location referencing is described in detail in the AGORA-C specification on describing “on-the-fly” location reference modeling.
Under the AGORA-C specification, encoding rules provide necessary semantics both for creating the location code at the sending system and for interpreting this code in the end terminal. Thus, the role of the encoding rules is both to provide constraints for selecting and creating the set of information elements at the sending system, and to provide a consistent interpretation basis for the receiving system to reconstruct the location reference as intended by the sending system.
Because the location code is dynamically coded at the sending end and dynamically decoded at the receiving end, a premium is placed upon limiting the amount of data needed to unambiguously identify a location. (A “location” is defined in ISO 17572 and AGORA-C and can be a series of connected road sections, where each road section is bounded by two different intersections (IS).) While some encoding schemes have been developed which conform to AGORA-C, some have been shown to be slow and not always successful.